The discovery by Schaudinn of the Spirochæta pallida was the starting-point of an extensive series of investigations which have thrown much light on the nature of syphilitic infection and its consequences. Reliable authorities are agreed as to the aetiological relationship between this organism and syphilitic disease, and its presence can with ease be demonstrated in chancres, syphilides, and the tissues of cases of congenital syphilis. In tertiary lesions its presence has been noted in gummata and in aortitis, though only in a very few cases. In the so-called para-syphilitic diseases it has not yet been seen. If it be the case that the presence of the organism be indispensable to the production of tertiary and para-syphilitic lesions, the difficulty of demonstrating it may be due to one or both of the following causes: (1) The organisms may be present in another form representing a different stage in their life cycle; or (2) they may be so few in number as to render demonstration extremely difficult, as is the case with the tubercle bacillus in the lesions of chronic fibroid phthisis. The difficulty consequent on a paucity in numbers may be enhanced by difficulty in staining. Two methods of staining the spirochete are in use–Giemsa's stain for film preparations, and a silver impregnation method for the examination of tissues. It is a remarkable fact that tissue which shows enormous numbers of silver impregnated organisms may show very few or none at all in the films stained by Giemsa's method, while on the other hand we have noted an extreme variability in the extent to which impregnation by silver may be obtained; for example, if the tissues of a syphilitic infant be fixed soon after death and while still in a very fresh state, impregnation may be very slight in some parts and distinct in others; thus, in the case of a syphilitic pneumonia, the organisms in the fresh proliferating pneumonic tissue are not seen, or are represented by delicate spirals which are recognised with difficulty, while in the desquamating and degenerating epithelium of the bronchi they may be much more distinct. It is, on the other hand, a striking fact that the spirochaetes in a syphilitic fætus which has been dead for some time are usually impregnated easily with silver, and this is in accord with other observations in the use of the silver impregnation method in demonstrating structures.